Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement

We calculate how the relic density of dark matter particles is altered when their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa interaction between the annihilating particles. Maintaining a dark matter abundance consistent with current observational bounds requires the normalization of the s-wave annihilation cross section to be decreased compared to a model without enhancement. The level of suppression depends on the specific parameters of the particle model, with the kinetic decoupling temperature having the most effect. We find that the cross section can be reduced by as much as an order of magnitude for extreme cases. We also compute the mu-type distortion of the CMB energy spectrum caused by energy injection from such Sommerfeld-enhanced annihilation. Our results indicate that in the vicinity of resonances, associated with bound states, distortions can be large enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the COBE/FIRAS experiment.Comment: 10 pages, 6 figures, accepted for publication in Physical Review D. Corrections to eqs. 9,10,14 and 16. Figures updated accordingly. No major changes to previous results. Website with online tools for Sommerfeld-related calculations can be found at http://www.mpa-garching.mpg.de/~vogelsma/sommerfeld

The Missing Satellite Problem in 3D

It is widely believed that the large discrepancy between the observed number of satellite galaxies and the predicted number of dark subhalos can be resolved via a variety of baryonic effects which suppress star formation in low mass halos.Supporting this hypothesis, numerous high resolution simulations with star formation, and associated feedback have been shown to reproduce the satellite luminosity function around Milky Way-mass simulated galaxies at redshift zero. However, a more stringent test of these models is their ability to simultaneously match the satellite luminosity functions of a range of host halo masses and redshifts. In this work we measure the luminosity function of faint (sub-Small Magellanic Cloud luminosity) satellites around hosts with stellar masses 10.5$<\log_{10}$M$_*$/M$_\odot<11.5$ to an unprecedented redshift of 1.5. This new measurement of the satellite luminosity function provides powerful new constraining power; we compare these results with predictions from four different simulations and show that although the models perform similarly over-all, no one model reproduces the satellite luminosity function reliably at all redshifts and host stellar masses. This result highlights the continued need for improvement in understanding the fundamental physics that governs satellite galaxy evolution.Comment: 9 pages, 2 figures, submitted to MNRA

Bound and unbound substructures in Galaxy-scale Dark Matter haloes

We analyse the coarse-grained phase-space structure of the six Galaxy-scale dark matter haloes of the Aquarius Project using a state-of-the-art 6D substructure finder. Within r_50, we find that about 35% of the mass is in identifiable substructures, predominantly tidal streams, but including about 14% in self-bound subhaloes. The slope of the differential substructure mass function is close to -2, which should be compared to around -1.9 for the population of self-bound subhaloes. Near r_50 about 60% of the mass is in substructures, with about 30% in self-bound subhaloes. The inner 35 kpc of the highest resolution simulation has only 0.5% of its mass in self-bound subhaloes, but 3.3% in detected substructure, again primarily tidal streams. The densest tidal streams near the solar position have a 3-D mass density about 1% of the local mean, and populate the high velocity tail of the velocity distribution.Comment: Submitted to MNRAS on 12/10/2010, 11 pages, 10 figure

Dark Matter Caustics

Caustics are a generic feature of the nonlinear growth of structure in the dark matter distribution. If the dark matter were absolutely cold, its mass density would diverge at caustics, and the integrated annihilation probability would also diverge for individual particles participating in them. For realistic dark matter candidates, this behaviour is regularised by small but non-zero initial thermal velocities. We present a mathematical treatment of evolution from Hot, Warm or Cold Dark Matter initial conditions which can be directly implemented in cosmological N-body codes. It allows the identification of caustics and the estimation of their annihilation radiation in fully general simulations of structure formation.Comment: 6 pages, 1 figure, Accepted for publication in MNRAS, minor edit

Ultra-fine dark matter structure in the Solar neighbourhood

The direct detection of dark matter on Earth depends crucially on its density and its velocity distribution on a milliparsec scale. Conventional N-body simulations are unable to access this scale, making the development of other approaches necessary. In this paper, we apply the method developed in Fantin et al. 2008 to a cosmologically-based merger tree, transforming it into a useful instrument to reproduce and analyse the merger history of a Milky Way-like system. The aim of the model is to investigate the implications of any ultra-fine structure for the current and next generation of directional dark matter detectors. We find that the velocity distribution of a Milky Way-like Galaxy is almost smooth, due to the overlap of many streams of particles generated by multiple mergers. Only the merger of a 10^10 Msun analyse can generate significant features in the ultra-local velocity distribution, detectable at the resolution attainable by current experiments.Comment: 9 pages, 6 figures, accepted for publication in MNRA

A Deep Learning Approach to Galaxy Cluster X-ray Masses

We present a machine-learning approach for estimating galaxy cluster masses from Chandra mock images. We utilize a Convolutional Neural Network (CNN), a deep machine learning tool commonly used in image recognition tasks. The CNN is trained and tested on our sample of 7,896 Chandra X-ray mock observations, which are based on 329 massive clusters from the IllustrisTNG simulation. Our CNN learns from a low resolution spatial distribution of photon counts and does not use spectral information. Despite our simplifying assumption to neglect spectral information, the resulting mass values estimated by the CNN exhibit small bias in comparison to the true masses of the simulated clusters (-0.02 dex) and reproduce the cluster masses with low intrinsic scatter, 8% in our best fold and 12% averaging over all. In contrast, a more standard core-excised luminosity method achieves 15-18% scatter. We interpret the results with an approach inspired by Google DeepDream and find that the CNN ignores the central regions of clusters, which are known to have high scatter with mass.Comment: 10 pages, 6 figures, accepted for publication in The Astrophysical Journa

Reproducing the kinematics of damped Lyman α systems

We examine the kinematic structure of Damped Lyman-alpha Systems (DLAs) in a series of cosmological hydrodynamic simulations using the AREPO code. We are able to match the distribution of velocity widths of associated low ionisation metal absorbers substantially better than earlier work. Our simulations produce a population of DLAs dominated by halos with virial velocities around 70 km/s, consistent with a picture of relatively small, faint objects. In addition, we reproduce the observed correlation between velocity width and metallicity and the equivalent width distribution of SiII. Some discrepancies of moderate statistical significance remain; too many of our spectra show absorption concentrated at the edge of the profile and there are slight differences in the exact shape of the velocity width distribution. We show that the improvement over previous work is mostly due to our strong feedback from star formation and our detailed modelling of the metal ionisation state

Nonspherical similarity solutions for dark halo formation

We carry out fully 3-dimensional simulations of evolution from self-similar, spherically symmetric linear perturbations of a Cold Dark Matter dominated Einstein-de Sitter universe. As a result of the radial orbit instability, the haloes which grow from such initial conditions are triaxial with major-to-minor axis ratios of order 3:1. They nevertheless grow approximately self-similarly in time. In all cases they have power-law density profiles and near-constant velocity anisotropy in their inner regions. Both the power-law index and the value of the velocity anisotropy depend on the similarity index of the initial conditions, the former as expected from simple scaling arguments. Halo structure is thus not "universal" but remembers the initial conditions. On larger scales the density and anisotropy profiles show two characteristic scales, corresponding to particles at first pericentre and at first apocentre after infall. They are well approximated by the NFW model only for one value of the similarity index. In contrast, at all radii within the outer caustic the pseudo phase-space density can be fit by a single power law with an index which depends only very weakly on the similarity index of the initial conditions. This behaviour is very similar to that found for haloes formed from LCDM initial conditions and so can be considered approximately universal.Comment: 8 pages, 7 figures, submitted to MNRA

Quenched fractions in the IllustrisTNG simulations: Comparison with observations and other theoretical models

We make an in-depth comparison of the IllustrisTNG cosmological simulations with observed quenched fractions of central and satellite galaxies, for Mstars = 109-12 M⊙ at 0 ≤ z ≤ 3. We show how measurement choices [aperture, quenched definition, and star formation rate (SFR) indicator time-scale], as well as sample selection issues (projection effects, satellite/central misclassification, and host mass distribution sampling), impact this comparison. The quenched definition produces differences of up to 70 (30) percentage points for centrals (satellites) above ∼1010.5 M⊙. At z Z 2, a larger aperture within which SFR is measured suppresses the quenched fractions by up to ∼50 percentage points. Proper consideration of the stellar and host mass distributions is crucial: Naive comparisons to volume-limited samples from simulations lead to misinterpretation of the quenched fractions as a function of redshift by up to 20 percentage points. Including observational uncertainties to theoretical values of Mstars and SFR changes the quenched fraction values and their trend and/or slope with mass. Taking projected rather than three-dimensional distances for satellites decreases the quenched fractions by up to 10 per cent. TNG produces quenched fractions for both centrals and satellites broadly consistent with observations and predicts up to ∼80 (90) per cent of quenched centrals at z = 0 (z = 2), in line with recent observations, and higher than other theoretical models. The quantitative agreement of TNG and Sloan Digital Sky Survey for satellite quenched fractions in groups and clusters depends strongly on the galaxy and host mass range. Our mock comparison highlights the importance of properly accounting for observational effects and biases

Constraining SIDM with halo shapes: Revisited predictions from realistic simulations of early-type galaxies

We study the effect of self-interacting dark matter (SIDM) and baryons on the shape of early-type galaxies (ETGs) and their dark matter haloes, comparing them to the predictions of the cold dark matter (CDM) scenario. We use five hydrodynamical zoom-in simulations of haloes hosting ETGs (Mvir sim 10 13 , M ⊙ and M ∗ ∼ 10 11 , M ⊙), simulated in CDM and a SIDM model with constant cross-section of σT/mχ = 1 cm2g-1. We measure the 3D and projected shapes of the dark matter haloes and their baryonic content using the inertia tensor and compare our measurements to the results of three HST samples of gravitational lenses and Chandra and XMM-Newton X-ray observations. We find that the inclusion of baryons greatly reduces the differences between CDM and a SIDM, together with the ability to draw constraints based on shapes. Lensing measurements reject the predictions of CDM dark-matter-only simulations and prefer one of the hydro scenarios. When we consider the total sample of lenses, observational data prefer the CDM hydro scenario. The shapes of the X-ray emitting gas are compatible with observational results in both hydro runs, with CDM predicting higher elongations only in the very centre. Contrary to previous claims at the scale of elliptical galaxies, we conclude that both CDM and our SIDM model can still explain observed shapes once we include baryons in the simulations. Our results demonstrate that this is essential to derive realistic constraints and that new simulations are needed to confirm and extend our findings